For a number of years, atactic polypropylene has been mixed with asphaltic bitumen for use in roofing materials and road paving. Atactic polypropylene, as used herein, is a substantially stereo irregular, non-crystalline polypropylene fraction that is usually produced as a by-product in the polymerization of propylene to form isotactic polypropylene, and which is separated from the isotactic material by solvent extraction. Depending on the industrial process, the soluble polymer fraction may also contain substantially amorphous ethylene/propylene copolymers and, in a lower amount, "tails" of isotactic polypropylene. This material is referred to herein as waste polypropylene.
Because of its non-crystalline structure atactic and waste polypropylene has not been widely used in the polymer industry. In recent years, however, several uses have been found for atactic polypropylene and it is now sold for use in mixing with asphalt to form strong, durable roofing materials, see for example U.S. Pat. No. 4,368,228 to Romolo Gorgati.
Since atactic and waste polypropylene is currently produced as a polymer by-product and is not a staple commercial product, the continued supply and consistency of this "waste" material is uncertain.